That’s super common for kernels (and generally super easy), so the kernel gets configured with the information it needs to run on the hardware for maximum speed. Let’s see, it looks like that just involves running make all and it does it all for you, so yeah pretty simple. It looks like it only handles 64-bit x86 systems so far but seems open to build on others once their bootstrapping processes are added, so unsure if it would work on an rpi currently. Ah, there is currently a GSoC project going on to add arm64 support, specifically to support the rpi, so although the rpi is not supported yet, it looks like it will be fairly ‘soon’.
Looks like a fun thing to mess with, still very incomplete but it seems they are trying to build a lot more than ‘just a kernel’, it has a windowing system and all.